16.1 - Sexual & Asexual Reproduction

Sexual and asexual reproduction

There are two ways living things reproduce: sexual and asexual.

Sexual reproduction

• Genetic information from two parents, a male and a female, is combined to produce offspring.
• Each parent produces special sex cells called gametes. Reproduction happens when the nucleus of a male gamete fuses with the nucleus of a female gamete, forming a single new cell called a zygote.
• Offspring receive a mixture of genetic information from both parents.
• They are therefore genetically different from each other and from either parent - this mixing leads to variation in the offspring.

Asexual reproduction

• There is only one parent. No gametes are involved and there is no joining of sex cells, so there is no mixing of genetic information.
• The parent produces new cells by mitosis - the type of cell division that makes genetically identical copies.
• Offspring are genetically identical to the parent and to one another. A group of genetically identical offspring produced this way are called clones.
• Examples:
  • Bacteria reproduce asexually by splitting in two
  • Many plants reproduce asexually too, e.g. a strawberry plant sending out runners that grow into new plants
  • Some animals can also reproduce this way

How gametes are made

• Gametes are produced by a type of cell division called meiosis.
• Meiosis halves the chromosome number, so each gamete carries half the genetic information.
• When two gametes join at fertilisation, the full chromosome number is restored.

Gametes - The sex cells

• A gamete is a sex cell - one of the special cells that join together in sexual reproduction.
• Gametes differ from ordinary body cells because they carry only half the genetic information.
• When a male gamete and a female gamete fuse, the full amount of genetic information is restored.
• Sperm and egg cells are examples of specialised cells, each shaped for its particular job in reproduction.

Gametes in animals

• Male gamete - The sperm cell, made in the male reproductive organs, the testes.
• Female gamete - The egg cell (also called the ovum), made in the female reproductive organs, the ovaries.

Gametes in plants

• Plants also reproduce sexually using gametes.
• Male gamete - Carried in a pollen grain.
• Female gamete - The egg cell found inside the flower.

Fertilisation - Forming the zygote and embryo

• Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete to form a single cell called a zygote.
• Each gamete nucleus is haploid - it contains a single set of chromosomes.
• When the two haploid nuclei fuse, the zygote nucleus becomes diploid - it contains two sets of chromosomes, with a pair of each type.
• So the two haploid gamete nuclei combine to restore the diploid number in the zygote.

Chromosome numbers in humans

• A human gamete is haploid, with one of each chromosome - 23 in total.
• A human diploid cell has 23 pairs - 46 chromosomes in total.

From zygote to embryo

• The zygote divides by mitosis, copying itself again and again to form a ball of cells.
• This ball of cells is the embryo - the earliest stage of a new individual, carrying a mixture of chromosomes from both parents.
• The ball of cells then implants into the lining of the uterus, where it continues to develop.

Adaptive features of gametes

The egg cell (female gamete) and the sperm cell (male gamete) are each adapted for their job in reproduction.

Adaptations of the egg cell

• Energy stores in the cytoplasm - Provide nutrients for the zygote, so it can divide and grow into an embryo after fertilisation.
• Jelly coat - A layer around the outside that changes structure the moment a sperm fertilises the egg. This stops any further sperm getting in, so the new individual ends up with the right amount of genetic material.

Adaptations of the sperm cell

• Flagellum - A long tail that lets the sperm swim towards the egg.
• Mitochondria - Present in large numbers, releasing the energy needed to move the flagellum.
• Acrosome - A region in the head holding enzymes that digest a way through the jelly coat of the egg cell, so the sperm can enter.

Egg cell vs sperm cell